Different regulatory modules of two mango ERS1 promoters modulate specific gene expression in response to phytohormones in transgenic model plants.

Ethylene is a key element of plant physiology, thus ethylene research is important for both, fundamental research and agriculture. Previous work on ethylene receptors focused on expression level and protein interaction, but knowledge on regulation of gene transcription is scarce. Promoters of mango ethylene receptor genes (pMiERS1a, pMiERS1b) were analysed particularly regarding responsiveness to hormones. The promoter sequences reveal some variation and they were characterized by identifying functional regulatory candidate modules via truncated-promoter approach. Based on ectopic gene expression studies in transgenic Arabidopsis and Nicotiana it is demonstrated that both promoters are positively responsive to ethylene. For pMiERS1a the AHBP/DOFF1 module is linked to ethylene responsiveness, while for pMiERS1b it is the module MYBL/OPAQ1. A negative gene regulation in response to abscisic acid (ABA) is linked to MYBL/DOFF2. A positive response to indole-3-acetic acid (IAA) was found for GTBX/MYCL1, containing the motifs IBOX/IDDF/TEFB, which are present in this combination only in pMiERS1b, but not in pMiERS1a. Conclusively, the general response of the ethylene receptor genes is conserved, but similar regulation can be linked to different modules. Further, a minor variation in a transcription factor binding site (TFBS) motif within an overall conserved module type can lead to a different expression.

Expression and interaction of the mango ethylene receptor MiETR1 and different receptor versions of MiERS1.

Different versions of the mango ethylene receptor MiERS1 were identified and the analysis indicates that, in addition to MiERS1, two short versions of this receptor (MiERS1m, MiERS1s), representing truncated proteins with central deletions of functional domains, are present in mango. The short receptor versions reveal a different expression pattern compared to MiERS1, and they are highly variably transcribed. With transient expression assays using fluorescent fusion proteins, the localisation and the interaction of the receptors were determined in leaf cells of the tobacco model. MiERS1, MiETR1, and the short MiERS1 receptor versions are anchored in the endoplasmic reticulum (ER) membrane and co-localise with each other and with an ER-marker. Furthermore, ectopic expression of the mango receptors appears to induce a re-organisation of the ER resulting in accumulation of ER bodies. Interaction assays suggest that both short MiERS1 receptor versions can bind to proteins located in the ER. Bi-molecular fluorescence complementation (BiFC) assays indicate, that MiERS1m may dimerise with itself and can also interact with MiERS1, but not with MiETR1. Further, it as found that MiETR1 can interact with MiERS1. Interaction of MiERS1s with the other ethylene receptors could not be detected, although it was located in the ER membrane system.

Ethephon induced abscission in mango: physiological fruitlet responses.

Fruitlet abscission of mango is typically very severe, causing considerable production losses worldwide. Consequently, a detailed physiological and molecular characterization of fruitlet abscission in mango is required to describe the onset and time-dependent course of this process. To identify the underlying key mechanisms of abscission, ethephon, an ethylene releasing substance, was applied at two concentrations (600 and 7200 ppm) during the midseason drop stage of mango. The abscission process is triggered by ethylene diffusing to the abscission zone where it binds to specific receptors and thereby activating several key physiological responses at the cellular level. The treatments reduced significantly the capacity of polar auxin transport through the pedicel at 1 day after treatment and thereafter when compared to untreated pedicels. The transcript levels of the ethylene receptor genes MiETR1 and MiERS1 were significantly upregulated in the pedicel and pericarp at 1, 2, and 3 days after the ethephon application with 7200 ppm, except for MiETR1 in the pedicel, when compared to untreated fruitlet. In contrast, ethephon applications with 600 ppm did not affect expression levels of MiETR1 in the pedicel and of MiERS1 in the pericarp; however, MiETR1 in the pericarp at day 2 and MiERS1 in the pedicel at days 2 and 3 were significantly upregulated over the controls. Moreover, two novel short versions of the MiERS1 were identified and detected more often in the pedicel of treated than untreated fruitlets at all sampling times. Sucrose concentration in the fruitlet pericarp was significantly reduced to the control at 2 days after both ethephon treatments. In conclusion, it is postulated that the ethephon-induced abscission process commences with a reduction of the polar auxin transport capacity in the pedicel, followed by an upregulation of ethylene receptors and finally a decrease of the sucrose concentration in the fruitlets.